Preheat the oven to 350F.
In a bowl, combine the beef, white onion, garlic, bread crumbs, red pepper flakes, parsley, and salt. Mix well.
Dust a clean work surface with a handful of flour and roll out the pastry to a 1/4-inch thickness. Spread Dijon mustard over one side of the puff pastry.
Add half of the beef mixture on the rolled out pastry sheet. Line up the hard-boiled eggs in a row down the center of the meat. Add the rest of the beef mixture to cover the eggs and shape it into a log. Roll it tucking the ends inside to create a large sausage shape.
Brush with the egg all over and place it on a baking sheet. Bake for 50 minutes-1 hour, or until the pastry is golden and the roll is cooked through.